Conventionally, a solenoid apparatus is used in a valve device such as a fuel injection valve and a pressure control valve. The solenoid apparatus includes a coil, which is supplied with electricity to operate a movable member. The solenoid apparatus has a fixed core and a movable core. The movable core is attracted to the fixed core by magnetic force generated therebetween when the coil is supplied with electricity. The movable member of the solenoid apparatus is operated together with the movable core.
The fixed core and the movable core repeat colliding against each other by repeating supplying electricity to the coil and repeating terminating the supplying electricity to the coil. According to U.S. Pat No. 5,732,888 (JP-A-8-506877), the movable core has a stepwise portion in the end thereof, in which the fixed core and the movable core oppose to each other, for securing a predetermined contact area therebetween. The stepwise portion is substantially in one of a cylindrical shape and a substantially tapered shape, for example. The fixed core and the movable core respectively have the ends thereof, in which the fixed core and the movable core oppose to each other. The ends of the fixed core and the movable core respectively have a hardened layer for reducing abrasion and deformation due to collision against each other.
In this structure of the solenoid apparatus, the movable core has the stepwise portion continuously extending in the circumferential periphery throughout in the circumferential direction thereof. In this structure, fluid intrudes into the gap between the fixed core and the movable core when the fixed core makes contact with the movable core. The fluid intruding between the fixed core and the movable core may cause squeeze force due to surface tension of the fluid, when the fixed core is spaced from the movable core. When the squeeze force arises, the squeeze force disturbs spacing motion of the fixed core from the movable core, consequently, response of the movable core may be degraded when the movable core is spaced from the fixed core.
As the fixed core repeats colliding against the movable core, the fixed core may fit to the movable core due to abrasion caused by colliding against each other. This fitting arises due to deformation caused in the end of the fixed core and the end of the movable core opposing to each other. As the end of the fixed core fits to the end of the movable core, the contact surface therebetween increases. Therefore, as the time elapses, the squeeze force arising between the fixed core and the movable core increases. As a result, response of the movable core when the movable core is spaced from the fixed core may change as the time elapses.
Furthermore, the hardening layer is formed in the ends of the fixed core and the movable core by hard chrome plating, for example. This plating process takes long, consequently, manufacturing process may be lengthened.
In addition, the end of the fixed core may not make contact with the end of the movable core via the entire surfaces therebetween in an initial condition thereof because of variation in manufacturing accuracy of the colliding portion and non-uniformity of the hardened layer. Accordingly, even when a hardened layer such as hardened chrome plated layer is formed, the end of the fixed core may not make contact with the end of the movable core via the entire surfaces therebetween in the initial condition thereof immediately after manufacturing the solenoid apparatus. However, as the fixed core repeats colliding against the movable core, the end of the fixed core gradually fits to the end of the movable core, so that the contact area therebetween may gradually increase. As a result, as time elapses, the response of the movable core may be degraded.